Methods and systems for laser calibration and eye tracker camera alignment

ABSTRACT

The present invention provides methods, systems, and apparatus for calibrating a laser ablation system, such as an excimer laser system for selectively ablating a cornea of a patient&#39;s eye. The invention also facilitates alignment of eye tracking cameras that measure a position of the eye during laser eye surgery. A calibration and alignment fixture for a scanning laser beam delivery system having eye tracking cameras may include a structure positionable in a treatment plane. The structure having a feature directing laser energy incident thereon to a calibration energy sensor, at least one reference-edge to determine a characteristic of the laser beam (shape, dimensions, etc.), and an artificial pupil to determine alignment of the eye tracking cameras with the laser system.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application is a divisional of and claims the benefit of priorityfrom U.S. patent application Ser. No. 10/685,253, filed Oct. 13, 2003,which is a divisional of and claims the benefit of priority from U.S.patent application Ser. No. 10/131,622, filed Apr. 23, 2002, now U.S.Pat. No. 6,666,855, which is a continuation-in-part of and claims thebenefit of priority from U.S. patent application Ser. No. 09/395,809,filed Sep. 14, 1999, now U.S. Pat. No. 6,559,934, the full disclosuresof which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

The present invention is generally directed to methods, systems, andapparatus for laser calibration and eye tracker camera alignment. Inparticular, the present invention relates to methods and systems formeasuring laser energy, shape, and dimensions of a laser beam from alaser beam delivery system, particularly opthalmological surgerysystems, and aligning eye tracking cameras used in conjunction with suchlaser systems that measure a position of the eye during laser eyesurgery.

Laser-based systems are now used in opthalmological surgery on cornealtissues to correct vision defects. These systems use lasers to achieve adesired change in corneal shape, with the laser removing thin layers ofcorneal tissue using a technique generally described as ablativephotodecomposition to alter the cornea's refractive power. Laser eyesurgery techniques are useful in procedures such as photorefractivekeratotomy (PRK), phototherapeutic keratectomy (PTK), laser in situkeratomileusis (LASIK), and the like.

In such laser-based systems and methods, the irradiated flux density andexposure time of the cornea to the laser radiation are controlled so asto provide a surface sculpting of the cornea to achieve a desiredultimate surface change in the cornea. To that end, ablation algorithmshave been developed that determine the approximate energy density thatmust be applied to remove a certain depth of tissue from the cornea. Atultraviolet wavelengths, for example, a cumulative energy density ofabout 1 joule/cm² will typically ablate corneal tissue to a depth ofabout one micron when applied in a series of pulses of about 40 to 400millijoules/cm². Accordingly, the ablation algorithms are tailored foreach procedure depending on the amount and the shape of corneal tissuewhich will be removed to correct a particular individual's refractiveerror.

In order to properly use these laser ablation algorithms, the laser beamdelivery system typically should be calibrated. Calibration of the lasersystem helps ensure removal of the intended shape and quantity of thecorneal tissue so as to provide the desired shape and refractive powermodification to the patient's cornea. For example, deviation from adesired laser beam shape or size, such as the laser beam exhibiting anon-symmetrical shape or an increased or decreased laser beam diameter,may result in tissue ablation at an undesired location on the patient'scornea which in turn leads to less than ideal corneal sculpting results.As such, it is beneficial to know the shape and size profiles of thelaser beam so as to accurately sculpt the patient's cornea through laserablation. In addition, it is usually desirable to test for acceptablelevels of system performance. For example, such tests can help ensurethat laser energy measurements are accurate. Ablations of plastic testmaterials are often performed prior to laser surgery to calibrate thelaser energy and ablation shape of the laser beam delivery system.Although such laser ablation calibration techniques are fairlyeffective, in some instances, alternative methods for laser energy andbeam shape calibration may be advantageous.

A variety of integrated structures have been proposed for both scanningof a laser beam across the corneal tissue and tracking of eye movements.Tracking of the eye during laser eye surgery has been proposed to avoiduncomfortable structures which attempt to achieve total immobilizationof the eye. Tracking further compensates for eye movement during atreatment procedure so that the intended portion of the eye may beaccurately ablated. An exemplary two camera off-axis eye tracker forlaser eye surgery is described in U.S. Pat. No. 6,322,216 B1, assignedto the assignee of the present application, the full disclosure of whichis incorporated herein by reference. In this system, first and secondcameras or image capture devices are oriented toward the eye. An energydelivery system laterally deflects an energy stream toward the cornealtissue along a first and second axis in response to movement of the eyesensed by the first and second image capture devices. Alignment of suchimage capture devices may be facilitated by a jig plate.

In light of the above, it would be desirable to provide improvedmethods, systems, and apparatus for calibrating laser energy, laser beamshape, and/or laser beam dimensions from a laser eye surgery system. Itwould be particularly desirable if such improvements enhancedcalibration accuracy without significantly increasing the overall systemcost and complexity. It would be further desirable if such methods,systems, and apparatus further allow for eye tracker camera alignment sothat laser calibration and camera alignment may be conveniently andeffectively carried out utilizing a single, reusable apparatus. At leastsome of these objectives will be met by the methods, systems, andapparatus of the present invention described hereinafter.

BRIEF SUMMARY OF THE INVENTION

The present invention provides methods, systems, and apparatus forcalibrating a laser ablation system, such as an excimer laser system forselectively ablating a cornea of a patient's eye. The invention alsofacilitates alignment of eye tracking cameras (which are often used inconjunction with such laser systems) that measure a position of the eyeduring laser eye surgery. In particular, the present invention providesmethods and systems which measure laser energy, laser beam shape, and/orlaser beam dimensions with enhanced calibration accuracy withoutsignificantly increasing the overall system cost and complexity.Moreover, the present invention allows for laser calibrations and eyetracker camera alignment to be effectively and conveniently carried out.

In a first aspect of the present invention, a method for calibratinglaser energy from a laser eye surgery system comprises transmitting orreflecting a laser beam suitable for ablation of corneal tissue from asurface, such as a galvanometric mirror having a reflecting surface or abeam splitter, and scanning the laser beam across a calibration fixturehaving a feature, such as an opening, reference-edge, or artificialpupil. Sample laser energy is separated from the beam at the surfaceduring the scanning and measured. Laser energy transmitted past ordirected at the feature during the scanning is measured. A calibrationof the laser system is then determined by comparing the energymeasurements. Energy measurements during the scanning are typically madeby energy sensors, such as a photodetectors, light detectors, energymeters, and like detectors that are positioned near, adjacent to, orbehind the surface or calibration fixture.

Calibration of the laser system is determined by comparing a ratio ofthe measured sample laser energy and the measured laser energy directedat the feature to a predetermined tolerance. If the ratio is within thepredetermined tolerance, calibration of the laser system furthercomprises independently comparing the measured sample laser energy to afirst threshold range and the measured laser energy directed at thefeature to a second threshold range. Calibration of the laser system iscomplete if the measured sample laser energy is within the firstthreshold range and the measured laser energy directed at the feature iswithin the second threshold range. In a passing calibration, thecalibration fixture may then be removed from a treatment plane and thelaser beam directed towards a patient's cornea for ablating the corneawith the calibrated system. However, if the measured sample laser energyis outside the first threshold range or the measured laser energydirected at the feature is outside the second threshold range, a faultis indicated in the laser system, such as flawed delivery system opticsor a flawed laser.

The laser beam may be transmitted from several different positions onthe surface. In the case where the ratio is outside the predeterminedtolerance, each ratio of the measured sample laser energy and themeasured laser energy directed at the feature is analyzed for each laserbeam position on the surface to determine if the ratio is positionindependent. If the ratio is not position independent, a fault isindicated either in the surface or an energy sensor that measures sampleenergy or laser energy directed at the feature. If the ratio is positionindependent, a fault is indicated either in the energy sensor thatmeasures sample energy or laser energy directed at the feature or thelaser beam delivery system. Calibration of the laser system may furtherindicate if an energy sensor measures the sample energy or laser energydirected at the feature at an accuracy within a predetermined threshold.

As described above, the surface preferably comprises a mirror having areflecting surface, wherein a photodetector measures sample energy, suchas laser energy leakage through the mirror. The feature comprises anopening in the calibration fixture which is positioned adjacent atreatment plane, wherein a photodetector measures laser light energypassing through the opening. A variation in each photodetector due tospatial non-uniformity is further measured prior to laser beam scanningacross the calibration fixture to separate this effect from the laserenergy calibration calculations described above. Moreover, a largenumber of measurements are made so that contributions due to detectornoise are relatively insignificant as compared to an average of laserenergy measurements. The tolerance and threshold values will depend onthe level of calibration accuracy desired. For example, thepredetermined ratio tolerance provides preferably 8% or less inaccuracy,more preferably 4% or less inaccuracy, most preferably 2% or lessinaccuracy while the threshold values may provide 1% or less inaccuracy.The laser beam will typically be oriented perpendicular to thecalibration fixture. The present methods advantageously allow forenhanced laser energy calibration as energy measurements from twophotodetectors are used to determine an accurate calibration of thelaser system. Moreover, energy measurements from two photodetectorsallow for fault detection within the laser system to be narrowed to aspecific source(s), which in turn facilitates fast and accurateadjustment of the laser system.

The calibration feature may further comprise a first reference-edge,such as a knife-edge, so as to determine a characteristic of the laserbeam by measuring laser energy passing the first reference-edge duringscanning with a photodetector. Multiple measurements are generated asthe laser beam is fully incident on the first reference-edge (i.e. thelaser beam is fully blocked from reaching the photodetector by thereference-edge) to the laser beam being fully incident on thephotodetector (i.e. the laser beam is not blocked by thereference-edge). The calibration feature will preferably comprise asecond reference-edge oriented at an angle relative to the firstreference-edge. A characteristic of the laser beam may be determined bymeasuring laser energy passing the second reference-edge duringscanning.

An intensity profile of the laser beam may be determined from themeasured laser energy passing the first or second reference-edges duringscanning. The scanning laser beam provides an integration of the laserbeam intensity profile. Dimensions of the laser beam may then bedetermined from the laser beam intensity profile. For example,dimensions of the laser beam may be determined by finding positions ofthe laser beam along the two orthogonal reference-edges where themeasured laser energy passing the reference-edge during the scanningreaches a certain percent of a maximum signal. In some instances, theintensity profile of the laser beam may be verified so that it is withina predetermined acceptable range from the compared energies. A shape ofthe laser beam may further be determined by measuring a rate of changeof the measured laser energy passing the reference-edge during scanning.Laser beam shape and dimension measurements provide information on beamquality, such as ellipticity, eccentricities, and asymmetries in thelaser beam, which in turn facilitates accurate sculpting of the cornea.

The calibration feature may further be imaged with an image capturedevice of an eye tracker system so as to align the image capture devicewith the laser system. In such instances, the calibration feature maycomprise four dark circles that preferably emulate eye pupils disposedat four corners of a square. Alternatively or additionally, the imagedfeature may comprise an opening or a reference-edge.

In another aspect of the present invention, methods for characterizing ascanning corneal ablation laser beam are provided. One method comprisesscanning a laser beam across a calibration fixture having areference-edge, measuring the laser beam energy passing thereference-edge while scanning the laser beam, and deriving acharacteristic of the laser beam from the measured laser beam energy.The calibration fixture may be removed following measurement and the apatient's eye treated by ablating the patient's cornea with the measuredlaser beam.

In yet another aspect of the present invention, systems for calibratinglaser energy from a laser beam delivery system are provided. Suchsystems may comprise a scanning laser beam delivery system, preferably alaser eye surgery system, a surface that directs laser energy from thelaser beam delivery system toward a treatment plane, the surfaceseparating a sample laser energy from the beam, a first photodetectorpositioned in a first optical path of the sample laser energy from thesurface, a calibration fixture positioned adjacent the treatment plane,and a second photodetector positioned in a second optical path of thelaser beam from the feature of the calibration fixture. The firstphotodetector emits a first output signal in response to the samplelaser energy, for example, the amount of laser energy leakage throughthe surface or mirror. The second photodetector emits a second outputsignal in response to the laser beam incident thereon. A processor isalso included in the system to determine a calibration of the lasersystem or a characteristic of the laser beam in response to the firstand second output signals.

The calibration feature may comprise an opening in the calibrationfixture that is sufficiently large so that a whole of the laser beam canpass through it. The calibration feature may further comprise areference-edge or two reference-edges so that the laser beam is directedfrom the surface across each reference-edge so as to determine acharacteristic of the laser beam (e.g., shape, dimension). In anexemplary embodiment, the calibration feature has a cross-like patterncomprising twelve reference-edges so as to allow for multiplemeasurements which in turn enhances laser beam dimension and shapemeasurements. The system may further comprise an image capture deviceorientated toward the treatment plane and an image processor coupled tothe image capture device. The image processor determines a position ofthe calibration fixture for verification of alignment between the imagecapture device and the laser delivery system. In such instances, theimaged calibration feature preferably comprises four dark circlesdisposed at four corners of a square pattern.

In a still further aspect of the present invention, a calibration andalignment fixture for a scanning laser beam delivery system having atleast one image capture device may comprise a structure positionable ina treatment plane. The structure has a feature directing laser energyincident thereon to a calibration energy sensor, at least onereference-edge to determine a characteristic of the laser beam (e.g.,shape, dimension), and an artificial pupil to determine alignment of theat least one image capture device with the laser system. The calibrationfeature comprises an opening that is sufficiently large so that a wholeof the laser beam can pass through it. Preferably, the calibrationfixture has two reference-edges, the second reference-edge oriented atan angle relative to the first reference-edge, more preferably thefixture has a cross-like pattern comprising twelve reference-edges. Theartificial pupil may comprise a dark circle, preferably four darkcircles disposed at four corners of a square, or an opening or hole inthe fixture. The cross-like pattern of twelve reference-edges may alsobe used to align the image capture device with the laser system.Conveniently, laser calibration and laser beam characteristics may bedetermined and eye tracking cameras aligned effectively via a single,reusable fixture.

In another aspect of the present invention, a method for calibrating alaser eye surgery system having eye tracking cameras is provided. Aposition of a laser beam is measured, a position of a calibrationfeature is measured, and the measured position of the laser beam iscompared to the measured position of the calibration feature. If themeasured positions are within a predetermined tolerance, the eye may betreated via corneal ablation.

A further understanding of the nature and advantages of the presentinvention will become apparent by reference to the remaining portions ofthe specification and drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a schematic of a system for laser calibration and eyetracker camera alignment constructed in accordance with the principlesof the present invention.

FIG. 2 illustrates an exploded view of the calibration and alignmentfixture employed in the system of FIG. 1.

FIGS. 2A and 2B illustrate alternative configurations of the calibrationand alignment fixture which may be employed in the system of FIG. 1.

FIG. 3 is a simplified block diagram illustrating a method forcalibrating laser energy from a laser beam delivery system employing thesystem of FIG. 1.

FIG. 4 illustrates another configuration of a calibration fixture whichmay be employed in the system of FIG. 1.

FIG. 5 is a perspective view of a laser beam being scanned over areference-edge of the calibration fixture of FIG. 4 at a moment in timewhen the laser beam is centered over the reference-edge.

FIGS. 6A-6C are sequential illustrations of the laser beam moving acrossthe reference-edge of FIG. 4.

FIG. 7 graphically illustrates output signals of the secondphotodetector during the scanning illustrated in FIGS. 6A-6C.

FIG. 8 is a top plan view of a laser beam being scanned over twoperpendicular reference-edges of the calibration fixture of FIG. 4.

FIG. 9 is a simplified perspective view of eye tracking cameras inconjunction with the calibration and alignment fixture of FIGS. 1 and 2.

FIGS. 10 and 11 illustrate an alternate embodiment of the calibrationand alignment fixture which may be employed in the system of FIG. 1.

FIG. 12 is a simplified block diagram illustrating a method forcalibrating a laser eye surgery system having eye tracking cameras.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides methods, systems, and apparatus forcalibrating a laser ablation system, such as an excimer laser system forselectively ablating a cornea of a patient's eye. The present inventionalso facilitates aligning eye tracking cameras that measure a positionof the eye during laser eye surgery. In particular, the presentinvention provides methods and systems which measure laser energy, laserbeam shape, and/or laser beam dimensions with enhanced calibrationaccuracy. By determining an exact quality of a laser beam, a desiredcorneal ablation treatment can be accurately effected via an ablationalgorithm without underablating or overablating corneal tissue, or thelaser beam becoming incident on undesired locations of corneal tissuecausing off-center ablations. Moreover, embodiments of the presentinvention allow for laser beam calibration and eye tracker cameraalignment to be simply and conveniently carried out utilizing a single,reusable fixture.

Referring now to FIG. 1, an exemplary calibration system 10 constructedin accordance with the principles of the present invention forcalibrating laser energy and aligning eye tracking cameras isschematically illustrated. System 10 is particularly useful forcalibrating and aligning a laser ablation system of the type used toablate a region of the cornea in a surgical procedure, such as anexcimer laser used in photorefractive keratotomy (PRK), phototherapeutickeratectomy (PTK), laser in situ keratomileusis (LASIK), and the like.Such systems 10 generally comprise a laser 11, a scanning laser beamdelivery system 12, a surface, such as, a mirror 14 having a reflectingsurface that directs laser energy 16 from the laser beam delivery system12 toward a treatment plane, a first photodetector 18 positioned behindthe mirror 14, a calibration fixture 20 positioned near or adjacent thetreatment plane, and a second photodetector 22 positioned behind thecalibration fixture 20. The first photodetector 18 provides a firstoutput signal 24 in response to sample laser energy separated from thebeam at the surface, in this instance, the amount of laser energyleakage through the mirror 14. The second photodetector 22 provides asecond output signal 26 in response to laser beam incident thereon andpassing by a feature, such as a feature formed in an opening 28, in thecalibration fixture 20. A computer system 30 is also included in thesystem 10 to record, process, and analyze the first and second outputsignals 24 and 26 to determine a calibration of the laser system or acharacteristic of the laser beam. The computer 30 may also providesignals 13 for controlling the laser 11 and laser beam delivery system12. Computer system 30 generally includes a processor, tangible mediafor storing instructions, random access memory, and other storage medialike hard and floppy drives. It will be appreciated that the followingdepictions are for illustration purposes only and does not necessarilyreflect the actual shape, size, or dimensions of the integratedcalibration and alignment system 10. This applies to all depictionshereinafter.

The laser beam delivery system 12 may include, but is not limited to, anexcimer laser such as an argon-fluoride excimer laser producing laserenergy with a wavelength of about 193 nm. Alternative laser systems mayinclude solid state lasers, such as frequency multiplied solid statelasers, flash-lamp and diode pumped solid state lasers, and the like.Exemplary solid state lasers include UV solid state lasers producingwavelengths of approximately 188-240 nm such as those disclosed in U.S.Pat. Nos. 5,144,630, and 5,742,626; and in Borsuztky et al., Tunable UVRadiation at Short Wavelengths (188-240 nm) Generated by FrequencyMixing in Lithium Borate, Appl. Phys. 61:529-532 (1995), the fulldisclosures of which are incorporated herein by reference. A variety ofalternative lasers might also be used. For example, a pulsed solid statelaser emitting infrared light energy may be used as described in U.S.Pat. Nos. 6,090,102 and 5,782,822, the full disclosures of which areincorporated herein by reference. The laser energy generally comprises abeam formed as a series of discrete laser pulses, and the pulses may beseparated into a plurality of beamlets as described in U.S. Pat. No.6,331,177, the full disclosure of which is incorporated herein byreference.

The calibration fixture opening 28 is sufficiently large so that a wholeof the laser beam can pass through it. The calibration fixture 20further comprises two reference-edges 32, 34 so that the laser beam isdirected from the mirror 14 across each reference-edge having the secondphotodetector 22 positioned therebehind so as to determine acharacteristic of the laser beam (e.g., shape, dimension). In theexemplary embodiment shown, the calibration fixture 20 has a cross-likepattern comprising twelve reference-edges so as to allow for multiplemeasurements which in turn enhances laser beam dimension and shapemeasurements. The system 10 may further comprise first and secondcameras or image capture devices 36 and 38 orientated toward thetreatment plane to track a position of an eye. In such instances, thecalibration fixture 20 further comprises four dark circles 40 thatpreferably emulate eye pupils disposed at four corners of a squarepattern so as to facilitate alignment of the cameras 36 and 38 with thelaser system.

Referring now to FIG. 2, an exploded view of the exemplary calibrationand alignment fixture 20 employed in the system 10 of FIG. 1 isillustrated. The fixture 20 comprises a structure positionable in atreatment plane. The structure 20 generally comprises a flat sided bodyhaving a credit card structure which has a width in the range from 10 mmto 50 mm, a length in the range from 10 mm to 50 mm, and a thickness inthe range from 0.1 mm to 5 mm. The structure 20 may be formed from avariety of materials, including metal, steel, silicon, crystals, or likematerials. The structure 20 has an opening 28, groove, notch, or slit toallow for laser energy calibration, at least one reference-edge,preferably two reference-edges 32, 34 that are oriented perpendicular toeach other so as to determine a characteristic of the laser beam, and anartificial pupil, preferably four dark circles 40 that are disposed atfour corners of a square pattern so as to determine an optical centerand rotational alignment of eye tracking cameras. The calibrationfixture opening 28 is preferably centered within the structure 20. Inthe exemplary embodiment, the calibration fixture forms a cross-likepattern comprising twelve reference-edges, preferably knife-edges, whichare referenced as RI through R12 in FIG. 2. The reference-edges aregenerally oriented perpendicular to one another and have a length in therange from 1 mm to 10 mm. The dark circles 40 disposed on the fixture 20will typically have a diameter in the range from 0.25 mm to 2 mm, and beformed from a variety of materials, including metals, steel, silicon, orlike materials. The alignment circles 40 will be sufficiently placedaway from the reference-edges R1 through R12, with the square havingside lengths 21 of about 14 mm. It will be appreciated that theartificial pupil may also comprise an opening or hole in the calibrationfixture. Conveniently, laser energy calibration and laser beamcharacteristics may be determined and eye tracking cameras alignedeffectively utilizing this single, reusable fixture 20, as described ingreater detail below.

Referring now to FIGS. 2A and 2B, alternate embodiments of thecalibration and alignment fixture 20 for the scanning beam deliverysystem having eye tracking cameras are illustrated. In FIG. 2A, aplurality of cross-like openings 28A-28D and reference-edges 32A-32D,34A-34D are formed in the fixture 20 to allow for laser energy and laserbeam shape calibrations. Openings 28A-28D may also function as alignmentpupils 40A-40D for the eye tracking cameras. FIG. 2B shows a pluralityof square openings 28A-28D and reference-edges 32A-32D, 34A-34D formedin the fixture 20 to allow for laser energy and laser beam shapecalibrations. Square openings 28A-28D may also function as alignmentpupils 40A-40D for the eye tracking cameras.

Referring now to FIG. 3, laser energy calibration is performed byutilizing measurements from the two energy detectors 18 and 22. Thefirst photodetector 18 is placed behind the mirror 14 that directs thelaser beam 16 to a treatment plane, and typically to the patient's eye.The first photodetector 18 measures the leakage of ultraviolet laserthrough the mirror 14 and may be referred to herein as the patientenergy detector (PED). The second photodetector 22 is placed adjacentthe calibration fixture 20 and it measures the pulse laser energy usedfor a given treatment procedure. This photodetector may be referred toherein as the treatment energy detector (TED). The mechanical fixture20, as described above, is placed adjacent the treatment plane and ispositioned relative to the beam delivery system 12 via a hinged supportarm or mechanism 21 that allows movement of the fixture in and out ofthe treatment plane. The TED is placed behind the calibration fixture20. The fixture 20 has opening 28 through which the whole of laser beam16 can pass through while the laser beam position is scanned over aspecified area.

In operation, laser energy calibration comprises the steps oftransmitting the laser beam 16 suitable for ablation of corneal tissuefrom the mirror 14, scanning the laser beam 16 through the opening 28 inthe fixture 20, and measuring the output signals of the first 18 andsecond 22 photodetectors during the scanning. The first photodetectormeasures the laser energy leakage through the mirror. The secondphotodetector measures the laser light energy passing through thefixture opening. A calibration of the system is determined by comparingthe energy measurements.

Multiple output signal measurements from the first 18 and second 22photodetectors are generated as the laser beam is transmitted fromseveral different positions on the fixed mirror 14 by moving the laserbeam 16 in a direction designated by reference number 42 (FIG. 1).Typically, the laser beam delivery system 12 will include scanningoptics for moving the laser beam 16 along a predetermined pathway. Insome instances, the mirror 14 may be attached to a gimbal so that therotating mirror may scan the laser beam across the calibration fixture.The laser beam 16 will typically be oriented perpendicular to the secondphotodetector 22 as the laser beam 16 is directed across the calibrationfixture 20. A large number of measurements are made so thatcontributions due to detector noise are relatively insignificant ascompared to an average of laser energy pulse measurements.

The computer system 30 records, processes, and analyzes the outputsignals 24 and 26 emitted from the first and second photodetectors 18and 22. An exemplary protocol for calibrating laser energy is depictedin block diagram fashion in FIG. 3. PED 18 and TED 22 values aremeasured at several positions of the laser beam 16 on the mirror 14. Theratio of output measurements, PED/TED, is then determined and thePED/TED ratio is then compared against a predetermined tolerance. If theratio is within the predetermined tolerance, the output measurement fromthe PED is independently compared against a first threshold range andthe output measurement from the TED is independently compared against asecond threshold range. If the output measurement from the PED is withinthe first threshold range and the output measurement from the TED iswithin the second threshold range, the calibration fixture 20 is removedfrom the treatment plane and the laser beam 16 directed towards apatient's cornea for a sculpting treatment as the laser energymeasurement is accurately calibrated.

However, if the output measurement from the PED is outside the firstthreshold range or the output measurement from the TED is outside thesecond threshold range, a fault is indicated in the laser beam deliverysystem 12, such as flawed delivery system optics or a flawed laser.Moreover, in the case where the PED/TED ratio is outside thepredetermined tolerance, each PED/TED ratio of the output measurementsfrom the first and second photodetectors is analyzed for each laser beamposition scanned on the mirror to check if the PED/TED ratio wasposition independent. If the PED/TED ratio is not position independent,a fault is indicated either in the mirror 14, PED 18, or TED 22. If thePED/TED ratio is position independent, a fault is indicated either inthe TED 18, PED 22, or laser beam delivery system 12. For example, oneof the photodetectors may have deteriorated or the delivery opticstransmission is off. Moreover, in all failed calibration scenariosdescribed above, an ablation test on plastic test material may befurther performed.

A variation in each photodetector 18, 22 due to spatial non-uniformityis measured prior to laser beam scanning with the calibration fixture toseparate this effect from the laser energy calibration analysisdescribed above. In particular, spatial non-uniformity may be measuredby scanning the laser beam over the two detectors, PED and TED, withoutthe fixture 20 to obtain a map of the PED/TED ratio over the twodimensional range of laser beam positions during scanning. For example,the laser beam may be scanned in 0.1 mm increments over a circular areahaving a diameter of approximately 10 mm. This sampling provides about8000 measurements of the PED/TED ratio and forms a PED/TED map. Thetolerance and threshold values will depend on the level of calibrationaccuracy desired. For example, the predetermined ratio toleranceprovides preferably 8% or less inaccuracy, more preferably 4% or lessinaccuracy, most preferably 2% or less inaccuracy while the first andsecond threshold values may provide 1% or less inaccuracy. The presentcalibration methods advantageously allow for enhanced laser calibrationas measurements from two photodetectors 18 and 22 are used to determinelaser calibration accuracy. Moreover, measurements from twophotodetectors allows for fault detection within the laser deliverysystem to be narrowed down to a specific component(s) of the system,which in turn facilitates fast and accurate adjustment of the lasersystem 10.

Referring now to FIG. 4, another configuration of a calibration fixture44 is illustrated. Such a fixture may be utilized for laser energymeasurement as well measuring laser beam shape and dimensions with thesystem of FIG. 1. FIG. 5 shows a perspective view of the laser beam 16which is directed downwardly towards the first reference-edge 32, withthe second photodetector 22 positioned therebehind (not shown), at amoment in time during the laser beam scanning when a center of the laserbeam 16 is positioned exactly at the edge of the first reference-edge32. The laser beam 16 is typically directed from the mirror 14 acrossthe first reference-edge 32 so that the output signal from the secondphotodetector 22 corresponds to an area of the laser beam incident onthe second photodetector (i.e. the part of the laser beam that is notblocked by the reference-edge 32) during the scanning. As illustrated inFIG. 5, in the case where the laser beam 16 has a circular shape, afirst half 46 of the laser beam 16 will be incident on the secondphotodetector 22 while a second half 48 of the laser beam 16 will beoccluded by the calibration fixture 20.

Referring now to FIGS. 6A through 6C, sequential movement of the laserbeam 16 during scanning across the first reference-edge and onto thesecond photodetector 22 is illustrated. Multiple output signalmeasurements are generated from the second photodetector 22 as the laserbeam 16 is fully incident on the calibration fixture 20 (i.e. the laserbeam is fully blocked from reaching the photodetector by thereference-edge), as shown in FIG. 6A, to the laser beam 16 being fullyincident (as denoted by reference numeral 50) on the secondphotodetector 22, as shown in FIG. 6C. FIG. 6B shows a first half 46 ofthe laser beam 16 incident on the second photodetector 22 while a secondhalf 48 of the laser beam 16 is incident on the calibration fixture 20.An average of multiple output signal readings reduces variations in thedata due to photodetector noise. By measuring the output of the secondphotodetector it is possible to determine intensity profile, dimensions,and shape of the laser beam during the scanning. By comparing themeasured energy signal of the second photodetector 22 to the firstenergy detector 18, variations in the energy emitted by each pulse ofthe laser 11 are rejected as common mode noise.

Referring now to FIG. 7, an intensity profile of the laser beam 16 maybe determined from the output signal S from the second photodetector 22taken over time during the scanning (FIGS. 6A-6C) of the laser beam 16across the first reference-edge 32 and onto photodetector 22. Theintensity of output signal S of the second photodetector 22 willcorrespond to energy in the area of laser beam 16 which is not blockedalong the first reference-edge 32 of calibration fixture 20 and istherefore directly incident on second photodetector. Specifically, theintensity of signal S can be represented as an integral of the laserbeam profile which is not blocked by the reference-edge. Such integralsfor a Gaussian pulse can be represented as follows:S=S ₀ ^(x)(laser beam intensity profile in 1D)dx orS=S ₀ ^(surface)(laser beam intensity profile in 2D)d(surface)or for a “top hat” pulse of amplitude A and diameter X_(o) in which theenergy distribution is substantially uniform across the cross-section ofthe pulse, as follows:$S = {2{AS}_{0}^{x}\sqrt{\left( \frac{x_{0}}{2} \right)^{2} - \left( {x - \frac{x_{0}}{2}} \right)^{2}}{dx}}$

Points P1, P2, and P3 in FIG. 7 illustrate the measured intensity ofoutput signal S corresponding to the integrated laser beam profile atthe moments in time when laser beam is positioned as shown in FIGS. 6A,6B, and 6C respectively. When laser beam 16 is positioned to be fullyoccluded by calibration fixture 20, the photodetector 22 will typicallyemit only a small signal intensity N, representing noise in the system.As laser beam 16 is scanned across the first reference-edge 32,progressively more of the area of the laser beam 16 will reach thesecond photodetector 22, increasing the intensity of the secondphotodetector's output signal S. When laser beam 16 reaches the positionillustrated in FIGS. 5 and 6B, such that the first half 22 of beam spot20 will be incident upon the photodetector, signal S will reachapproximately ½ of its maximum signal intensity at point P2. Finally,when laser beam 16 eventually reaches the position illustrated in FIG.6C, such that the entire laser beam 16 is incident upon photodetector22, signal S will reach its maximum signal intensity at point P3. Hence,for a generally circular laser beam 16, the intensity of output signal Swill be in the shape of an S-shaped curve as shown in FIG. 7. In anexemplary embodiment, the signal 26 measured by detector 22 is dividedby the signal 24 from detector 18 to reject common mode noise from pulseto pulse energy variations in the laser beam 16 emitted from laser 11.This divided signal is preferably normalized and plotted as shown inFIG. 7. The laser beam intensity profile is then determined from ans-shaped curve of the normalized values.

Dimensions of the laser beam may then be determined from the laser beamintensity profile. A shape of the laser beam 16 may further bedetermined by measuring a rate of change of the output signal S from thesecond photodetector 22 during scanning. Laser beam shape and dimensionmeasurements provide information on beam quality, such as ellipticity,eccentricities, and asymmetries in the laser beam.

Referring now to FIG. 8, a preferred method for determining dimensions,shape, intensity, and position of the laser beam is illustrated. Themethod comprises scanning the laser beam 16 in a first direction D1across the first reference-edge 32 followed by scanning the laser beamin a second direction D2 across the second reference-edge 34 oriented atan angle to the first reference-edge 32, wherein the photodetector 22 ispositioned behind the first and second reference-edges 32 and 34. Anoutput signal from the photodetector 22 is measured during the scanning,the output signal corresponding to an area of the laser beam 16 incidenton the photodetector 22 during the scanning. Preferably, a signal fromphotodetector 18 is measured to reject common mode noise as describedabove. Scanning along two orthogonal directions allows for twodimensional measurements which in turn enhances laser beam dimension andshape measurements. A characteristic of the laser beam may be derivedfrom the laser beam energy measured by the photodetector.

One analysis method is to compute first four moments of the laser beamintensity profiles projected along the two orthogonal axes D1 and D2. Asmentioned above, the reference-edge setup provides integration of theseintensity profiles. Mathematically the moments of the beam intensityprofiles can be calculated using the integrated profile. For example,the measured moments may be compared against moments of ideal Gaussiandistribution. The difference between the two provides information aboutthe beam quality, such as laser beam diameter and/or shape.

Referring now to FIG. 9, the fixture 20 may further allow alignment ofthe horizontal and vertical eye tracker cameras 36 and 38, so as to zerox and y positions provided by the cameras, and so as to orient thecameras properly about their optical axes. A structure 52 holds thefixture 20 at the desired position during calibration. To provideadjustability, the cameras are mounted so as to have three axes ofrotation. Adjustments about these axes will preferably be provided byfine screw adjustment mechanisms with lock-downs provided to secure theposition of the camera once at the desired position and orientation.

Exemplary eye tracker cameras 36, 38 for laser eye surgery are describedin U.S. Pat. No. 6,322,216 B1, assigned to the assignee of the presentapplication, the full disclosure of which is incorporated herein byreference. Generally, first and second cameras or image capture devicesare oriented toward the eye. The energy delivery system laterallydeflects the energy stream toward the corneal tissue along a first andsecond axis in response to movement of the eye sensed by the first andsecond image capture devices. The horizontal and vertical cameras willoften comprise commercially available tracking systems such as thoseavailable from Iscan, Inc. of Burlington, Mass., or other comparablesystems.

The fixture 20 will have a pattern on a surface thereof comprising fourdark circles 40 that emulate eye pupils disposed at four corners of asquare. The dark circles 40 may be imaged by the eye tracking cameras 36and 38 so as to align the image capture devices with the laser system.Typically, an electronic cross-hair serves as the camera's reference.The electronic cross-hair is aligned both rotationally and in the x,yplanes with the fixture 20. The dark circles 40 are used for scalecalibration. The dark circles 40 are 14 mm apart. The eye tracker cameralocates the dark circles 40 and measures the number of pixels betweenthe circles. The scale factor is 14 mm/number of pixels (mm/pixel). Thedark circles can be used to confirm the optical center and rotationalalignment of the cameras.

Referring now to FIGS. 10 and 11, an alternate embodiment of calibrationfixture 100 for use with an eye tracking and laser system employs asingle aperture 102 formed in the fixture 100. The fixture 100 is formedfrom a material that does not transmit the laser beam. The aperture 102emulates a pupil of an eye and has a diameter in the range from about 2mm to 12 mm, preferably from about 3 mm to 9 mm, and more preferablyfrom about 4 mm to 8 mm. Alternatively, the aperture 102 may emulateanother structure of an eye, for example a limbus of the eye. Aphotodetector 22 measures the laser beam energy passing through theaperture 102 as the laser beam scans across the fixture 100. The laserbeam profile is measured as illustrated in FIG. 11. The position oflaser beam 16 is scanned over aperture 102. Aperture 102 includesvertical reference-edges 32A and 32B that are approximatelyperpendicular to horizontal reference-edges 34A and 34B. Alternatively,non-perpendicular reference edges may be used. Laser beam positions 110Ato 110L include positions intended to fully block and fully transmitlaser beam 16. The laser beam intensity profile may be determined fromthe signal output of the photodetector 22 during scanning acrosspositions 110A to 110L. Also, the laser beam profile may be calculatedby comparing measured energy levels to expected energy levels for thelaser beam blocked by slightly curved reference-edges 32A, 32B, 34A and34B. Opening 102 may alternatively function as an alignment pupil orlimbus for the eye tracking cameras.

Referring now to FIG. 12, a preferred method of testing alignment of alaser beam system and eye tracking system is illustrated. A laser beamposition is measured from the reference-edge setup described above.Alternatively, the laser beam position may be measured as described inU.S. Pat. No. 5,928,221, the full disclosure of which is incorporatedherein by reference. The measured laser beam position is stored in thememory of computer 30. The position of an artificial pupil is measuredas described above. Alternatively, a position of another artificialstructure that is optically similar to another structure of an eye, suchas a limbus, may be measured. For example, in the case of contrasttracking of the limbus, the artificial structure comprises a contrastboundary optically similar to at least a portion of a limbal boundaryformed between a scleral tissue structure and a corneal tissuestructure. The measured artificial eye structure position is stored in amemory of computer 30. The measurement of the artificial eye structureand laser beam may be sequential and include different calibrationtargets for measuring the artificial eye structure and measuring thelaser beam. The measured position of the artificial eye structure iscompared to the measured position of the laser beam. If the measuredpositions of the artificial eye structure and laser beam are within apredetermined threshold amount, for example 0.1 mm, a patient istreated. If the measured positions of the artificial eye structure andlaser beam are greater than a threshold amount, the system is calibratedif it has not been calibrated in the last 4 hours. The comparison of themeasured artificial eye structure and measured laser beam positions isrepeated in response to the system not having been calibrated within 4hours. If the system has been calibrated within the last 4 hours, thesystem is serviced.

Although certain preferred embodiments and methods have been disclosedherein, it will be apparent from the foregoing disclosure to thoseskilled in the art that variations and modifications of such embodimentsand methods may be made without departing from the true spirit and scopeof the invention. Therefore, the above description should not be takenas limiting the scope of the invention which is defined by the appendedclaims.

1. A method for characterizing a scanning corneal ablation laser beam,the method comprising: scanning a laser beam across a calibrationfixture having a reference-edge; measuring the laser beam energy passingthe reference-edge while scanning the laser beam; deriving acharacteristic of the laser beam from the measured laser beam energy;and removing the calibration fixture and ablating a patient's corneawith the measured laser beam.
 2. A calibration and alignment fixture fora scanning laser beam delivery system having at least one image capturedevice, the fixture comprising: a structure positionable in a treatmentplane, the structure having a feature selectively passing laser energyincident thereon to a calibration energy sensor, at least onereference-edge to determine a characteristic of the laser beam, and anartificial pupil to determine alignment of the at least one imagecapture device with the laser system.
 3. A fixture as in claim 2,wherein the feature comprises an opening, the opening being sufficientlylarge that a whole of the laser beam can pass through it.
 4. Anapparatus for calibrating a laser beam delivery system, comprising: atarget which is adapted to fluoresce when the laser beam is incidentthereon; a laser beam targeting system for directing the laser beamtowards the target; targeting optics for viewing the position of thetarget when the target fluoresces in response to the laser beam incidentthereon, the targeting optics defining a target position; and anadjustment mechanism for aligning the laser beam targeting system withthe target position.
 5. The apparatus of claim 4, wherein, the laserbeam targeting system comprises a reticle.
 6. The apparatus of claim 4,wherein the targeting optics for viewing the position of the targetcomprise a microscope.